Fail Fast on Missing or Invalid Inputs

Is your feature request related to a problem? Please describe. Yes. It's frustrating that PyInstaller begins the bundling process—which can be very time-consuming—only to fail at a much later stage due to issues like a missing image or an invalid file path. This results in a significant waste of time, especially when the error could have been caught with a quick pre-check.

Describe the solution you'd like I'd like PyInstaller to validate inputs and paths (such as images, binaries, and data files) before starting the actual build process. Essentially, it should implement a “fail fast” mechanism that checks the availability and validity of all specified resources up front, and halts early with a clear error message if something is wrong.

Describe alternatives you've considered

• Manually checking all paths and files before running PyInstaller, but this is error-prone and inefficient.
• Writing custom scripts to validate the input spec file and its assets before calling PyInstaller.

Additional context The current behavior can be especially painful in larger projects, where a full build can take several minutes. A preflight validation step would significantly improve the developer experience and reduce turnaround time during packaging iterations.